Transcript Document

Wireless Mesh Networks
By Cunqing Hua
The notes of this talk are excerpted from the lecture notes by
Prof. Akyildiz at Georgia Institute of Technology
References
• Faccin, S.M.; Wijting, C.; Kenckt, J.; Damle, A., Mesh WLAN
networks: concept and system design, IEEE Wireless
Communication, Vol 13, No. 2, 2006.
• Lee, M.J.; Jianliang Zheng; Young-Bae Ko; Shrestha, D.M.,
Emerging standards for wireless mesh technology, IEEE
Wireless Communication, Vol 13, No. 2, 2006
• Akyildiz, I.F., Wang, X. and Wang, W., Wireless Mesh Networks:
A Survey, Computer Networks Journal (Elsevier), March 2005.
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Outline
• Application Scenarios
• Network Architecture
• Characteristics
• Protocols Design
• Standardization Activities
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Wireless Mesh Networks
• Wireless Mesh Networks (WMN) are the networks in which
each node can communicate directly with one or more peer
nodes.
• Different from traditional wireless networks (e.g. 802.11
WLANs) requiring centralized access points to mediate the
wireless connection.
• Each node operates not only as a host but also as a router,
forwarding packets on behalf of other nodes that may not
be within direct wireless transmission range of their
destinations.
• It is dynamically self-organized and self-configured, nodes
can automatically establishing and maintaining mesh
connectivity among nodes
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Application Scenarios
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Broadband Home Networking
Community and Neighborhood Networking
Enterprising Networking
Metropolitan Area Networking
Transportation Systems
Building Automation
Health and Medical Systems
Security and Surveillance Systems
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Broadband Home Networking
• Current home network realized through IEEE 802.11 WLANs
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Problem  location of the access points.
Homes have many dead zones without service coverage.
Site survey are expensive and not practical
Installation of multiple access points is also expensive and not
convenient.
– Communications between nodes under two different access points
have to go through the access hub, not an efficient solution.
WMNs can resolve all these
issues in home networking!!!
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Community and Neighborhood Networking
Community networks based on cable, DSL and last-hop
wireless
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All traffic must flow through Internet, this significantly
reduces network resource utilization.
Large percentage of areas in between houses is not covered
by wireless services.
Gateways may not be shared and wireless services must be
set up individually, network service costs may increase.
Each home has single path to access Internet
WMNs can mitigate these disadvantages
and provide many applications such as
distributed file storage, distributed file
access, and video streaming.
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Enterprise Networking
• IEEE 802.11 WLANs
– Isolated islands, connections among them are achieved
through wired Ethernet
– Adding more backhaul access modems only increases
capacity locally, but does not improve robustness to link
failures, network congestion and other problems of the
entire enterprise network.
• WMNs Solutions
– Multiple backhaul access
modems can be shared by all
nodes in the entire network
– Scalable
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Metropolitan Area Networks
• WMNs provide higher transmission rate than cellular networks,
• The communication between nodes does not rely on a wired
backbone.
• An economic alternative to broadband networking
• Covers larger area than home, enterprise, building, or
community networks.
• Higher scalability
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Transportation Systems
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WMNs can extend access from stations and stops into buses, ferries,
and trains.
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Convenient passenger information services, remote monitoring of invehicle security video, and driver communications.
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Two key techniques are needed
– High-speed mobile backhaul from a vehicle to the Internet
– Mobile mesh networks within the vehicle.
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Building Automation
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Various electrical devices need to be controlled
and monitored.
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Standard wired networks is very expensive
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Wi-Fi networks can reduce the cost of such
networks. However, the deployment of Wi-Fis
for this application is still expensive.
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Low deployment cost of BACnet (Building
Automation and Control Networks) with WMNs
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Health and Medical Systems
• Monitoring and diagnosis data need to be processed and
transmitted across rooms for various purposes.
• Large data volume by high resolution medical images, various
periodical monitoring information
• Wi-Fi based networks must rely on the existence of Ethernet
connections, cause high system cost, complexity and dead spots.
• However, these issues do not exist in WMNs.
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Security and Surveillance
Systems
• Security surveillance systems is necessity for enterprise
buildings, shopping malls, grocery stores, etc.
• Still images and videos are the major traffic flowing in the
network, this application demands much higher network capacity
than other applications.
• WMNs are a much more viable solution than wired networks to
connect all devices.
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Network Architecture
WMNs consist of two types of nodes: Mesh Routers and Mesh Clients
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Mesh router
– Additional routing functions to support mesh networking.
– Multiple wireless interfaces with same or different wireless access
technologies.
– The gateway/bridge functionalities enable the integration of WMNs with
existing wireless networks(cellular, sensornet, Wi-Fi, WiMAX).
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Mesh Clients
– Conventional nodes (e.g., desktops, laptops, PDAs, PocketPCs, phones,
etc.) equipped with wireless network interface cards (NICs), and can
connect directly to wireless mesh routers.
– Customers without wireless NICs can access WMNs by connecting to
wireless mesh routers through, e.g., Ethernet.
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WMN Routers
Examples of mesh routers based on different embedded
systems: (a) PowerPC and (b) Advanced Risc Machines (ARM)
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WMN Clients
Examples of mesh clients: (a) Laptop, (b) PDA,
(c) Wi-Fi IP Phone and (d) Wi-Fi RFID Reader.
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WMN Architecture Classifications
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Infrastructure Meshing
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Client Mesh Networking
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Hybrid Mesh Networking
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Infrastructure Meshing
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Mesh routers form an mesh infrastructure among themselves.
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Provides backbone for clients and enables integration of WMNs with
existing wireless networks and Internet through gateway/bridge
functionalities.
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Clients connect to mesh router with wireless link or Ethernet
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Client WMNs
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Client nodes constitute peer-to-peer network, and perform routing and
configuration functionalities as well as provide end-user applications to
customers, mesh routers are not required.
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Multi-hop routing.
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Client nodes have to perform additional functions such as routing and selfconfiguration.
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Hybrid WMNs
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A combination of infrastructure and client meshing.
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Infrastructure provides connectivity to other networks such as the Internet, WiFi, WiMAX, cellular, and sensor networks;
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Mesh clients can access the network through mesh routers as well as directly
meshing with other mesh clients.
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The routing capabilities of clients provide better connectivity and coverage
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WMNs Characteristics
• Multi-hop wireless networks
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Support for Ad Hoc networking, and capability of self-forming,
self-healing, and self-organization
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Mobility dependence on the type of mesh nodes
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Multiple types of network access
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Dependence of power-consumption constraints on the type of
mesh nodes
Compatibility and interoperability with existing wireless networks
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Protocol Design
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Physical Layer
Mac Layer
Network Layer
Transport Layer
Application Layer
Network Management
Security
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Physical Layer Technologies
• Orthogonal frequency multiple access (OFDM) has significantly
increased the speed of IEEE 802.11 from 11 mbps to 54 mbps.
• Ultra-wide band (UWB) can achieve much higher rate for shortdistance applications.
• MIMO can increase system capacity by three times or even
more.
• Frequency agile or cognitive radios can achieve much better
spectrum utilization.
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Physical Layer: Research Issues
• Improve the transmission rate and the performance
of physical layer techniques
– OFDM, UWB
– Multiple-antenna systems
– Frequency agile
• Design higher layer protocols to utilize the advanced
features provided by physical layers
– MAC protocols for directional and smart antennas
– MAC protocols for MIMO systems
– Communication protocols for cognitive radios
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MAC Layer
Differences between WMNs MACs and Wireless
Networks MACs
• MACs for WMNs are concerned with more than one hop
communication
• MAC must be distributed and collaborative, and must
work for multipoint-to-multipoint communication.
• Network self-organization is needed for better collaboration
between neighboring nodes and nodes in multi-hop distances.
• Mobility affects the performance of MAC.
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Single Channel MACs
• Improving Existing MAC Protocols
– Adjust parameters of CSMA/CA
– Only achieve a low end-to-end throughput.
• Cross-layer design with advanced physical layer
techniques
– MAC based on directional antenna can eliminate exposed nodes,
but may introduce more hidden nodes
– MAC with power control can reduce exposed nodes, improve
spatial-reuse, but hidden nodes still exist
• Proposing Innovative MAC Protocols
– Revisiting MAC protocols based on TDMA or CDMA
– Design complexity and cost.
– Compatibility with existing MAC protocols
Not scalable, available bandwidth ~(1/2)^n
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Multi-Channel MACs
• Multi-Channel Single-Transceiver MAC
– Only one channel is active in each node, different nodes can
use different channels.
– Need to coordinate transmissions between nodes
• Multi-Channel Multi-Transceiver MACs
– Multiple parallel RF front-end chips and baseband processing.
– One MAC layer module to coordinate multiple channels.
• Multi-Radio MACs
– Multiple radios, each with its own MAC and physical layers.
– Communications in these radios are totally independent.
– A virtual MAC protocol to coordinate communications in all
channels.
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MAC Layer : Research Issues
• Scalable Single-Channel MACs
– Distributed and collaborative schemes to ensure scalability.
• Scalable Multi-Channel MACs
– Overall performance improvement in multiple channel
• Network Integration in the MAC Layer
– Advanced bridging functions in the MAC layer so that different
wireless radios can seamlessly work together.
– Reconfigurable/software radios may be the ultimate solution to
these bridging functions.
• MAC Protocol Implementation
– Modifying functions in the firmware or hardware is much more
complicated and costly.
– New architecture such that MAC functions can be completely
implemented in the software.
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Routing Layer
Features of routing protocol for WMNs:
• Multiple Performance Metrics
– Hop-count is not an effective routing metric.
– Other performance metrics, e.g., link quality and round
trip time (RTT), must be considered.
• Scalability
– Routing setup in large network is time consuming.
– Node states on the path may change.
– Scalability of routing protocol is critical in WMNs.
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Routing Layer
• Robustness
– WMNs must be robust to link failures or congestion.
– Routing protocols need to be fault tolerant with link failures
and can achieve load balancing.
• Adaptive Support of Both Mesh Routers and Mesh
Clients
– Mesh routers : minimal mobility, no constraint of power
consumption, routing is simpler
– Mesh clients : mobility, power efficiency, routing is complicated
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Need to design a routing protocol that can adaptively support both
mesh routers and mesh clients.
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Destination-Sequenced DistanceVector (DSDV)
• Proactive Protocols
• Each node maintains a routing table which stores
– next hop towards each destination
– a cost metric for the path to each destination
– a destination sequence number that is created by the destination
itself
– Sequence numbers used to avoid formation of loops
• Each node periodically forwards the routing table to its
neighbors
– Each node increments and appends its sequence number when
sending its local routing table
– This sequence number will be attached to route entries created for
this node
• DSDV in WMNs
– Supporting multidimensional cost metrics (QoS, power efficiency,
security, etc)
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DSDV Protocol
• Assume that node X receives routing
information from Y about a route to node Z
X
Y
Z
• Let S(X) and S(Y) denote the destination
sequence number for node Z as stored at
node X, and as sent by node Y with its routing
table to node X, respectively
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DSDV Protocol
• Node X takes the following steps:
X
Y
Z
– If S(X) > S(Y), then X ignores the routing information
received from Y
– If S(X) = S(Y), and cost of going through Y is smaller than
the route known to X, then X sets Y as the next hop to Z
– If S(X) < S(Y), then X sets Y as the next hop to Z, and S(X)
is updated to equal S(Y)
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Routing Layer- Research Issues
• Scalability
– Hierarchical routing protocols can only partially solve
this problem
– Geographic routing relies positioning technologies.
– New scalable routing protocols need to be developed.
• Better Performance Metrics
– New performance metrics need to be developed.
– Need to integrate multiple performance metrics into a
routing protocol
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Routing Layer - Research Issues
• Routing/MAC Cross-Layer Design
– Needs to interact with the MAC layer, e.g. adopting multiple
performance metrics from MAC layer.
– Merely exchanging parameters between them is not enough, merging
certain functions of MAC and routing protocols is a promising approach.
– For multi-radio or multi-channel routing, the channel/radio selection in
the MAC layer can help the path selection in the routing layer.
• Hybrid Routing
– Mesh routers and mesh clients have different constraints in power
efficiency and mobility.
– Need to adaptively support mesh routers and mesh clients.
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Transport Layer: Research Issues
• Cross-layer Solution to Network Asymmetry
– Routing protocol can select an optimal path for both data and ACK
packets.
– MAC layer and error control may need to treat TCP data and ACK
packets differently.
• Adaptive TCP
– WMNs will be integrated with the Internet and various wireless
networks such as IEEE 802.11, 802.16, 802.15, etc.
– Same TCP is not effective for all networks.
– Applying different TCPs in different networks is a complicated
and costly approach, and cannot achieve satisfactory performance.
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Application Layer
Applications supported by WMNs:
• Internet Access
– Advantages of WMNs: low cost, higher speed, and easy
installation.
• Distributed Information Storage and Sharing
– Data sharing between nodes within WMNs
– Query/retrieve information located in distributed database
servers.
• Information Exchange across Multiple Wireless Networks.
– Cellular phone talks Wi-Fi phone through WMNs,
– Wi-Fi user monitors the status of wireless sensor networks.
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Application Layer: Research
Issues
• Improve Existing Application Layer Protocols.
– Lower layers protocols cannot provide perfect support for the
application layer.
– E.g., packet loss and packet delay with a large jitter may fail many
Internet applications
– Existing application layer protocols need to be improved.
• New Application Layer Protocols for Distributed
Information Sharing.
– P2P protocols on the Internet may not perform well in WMNs,
– New application layer protocols need to be developed.
• Develop Innovative Applications for WMNs
– Applications cannot achieve best performance without WMNs.
– Enable WMNs to be a unique networking solution instead of just
another option of wireless networking.
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Network Management Protocols
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Mobility Management
Distributed scheme for WMNs can be simpler because the
existence of backbone nodes
Take advantages of the network backbone to design a lightweight distributed mobility management scheme for WMNs.
Location service is a desired feature by WMNs.
Power Management
For mesh routers, power management aims to control
connectivity, interference, spectrum spatial-reuse, and
topology.
For mesh clients, protocols should be power efficient.
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Network Management Protocols
• Network Monitoring
– Report statistics in the MIB to one or several servers.
– Data processing algorithms analyze these statistical data and
determine potential abnormality.
– To reduce overhead, schemes for efficient transmission of
network monitoring information are expected.
– To accurately detect abnormal operation and quickly derive
network topology of WMNs, effective data processing
algorithms need to be developed.
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Security
• WMNs lack efficient and scalable security solutions
– Distributed network architecture
– Vulnerability of channels and nodes in the shared wireless
medium
– Dynamic change of network topology.
• Two strategies
– Embedding security mechanism into network protocols
– Developing security monitoring response systems
– How to design and implement a practical security system,
including cross-layer secure network protocols and various
intrusion detection algorithms, is a challenging research
topic.
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WMNs Standards
• WPAN: Bluetooth, Zigbee
• WiFi: 802.11a, b, g, n
• WiMAX: 802.16
Range
50Km
100m
WiMAX
WPAN
100kb 1Mb
Wi-Fi
10Mb
100Mb
Data Rate
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WMNs Standards
• IEEE 802.16a WMAN Mesh
– “mesh mode” in addition to the point-to-multipoint(PMP)
mode defined in IEEE 802.16.
– Operating in the licensed and unlicensed lower
frequencies of 2–11 GHz, allowing non-line-of-sight
(NLO) communications, spanning up to a 50 km range.
– Supporting multihop communications.
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WMNs Standards
• 802.11s WLAN Mesh
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Multi-hop capability added to 802.11g/a/b
Auto configure on power up
Multi-channel multi-radio operation
Topology discovery
MAC Path selection protocol
Modified forwarding for QOS and mesh control
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WMNs Standards
• 802.11s MCF Sublayer
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